If it's true that we use something more when we enjoy using it, then it's also fair to say that a little bit of delight can go a long way in increasing usability.

I couldn't agree more with this. The little touches seem superfluous when taken in the context of pure function (i.e., "getting the job done"), but I surely appreciate them. Case in point: I've been using Twitterrific on the iPhone a lot the last couple days. I absolutely adore the pull-to-refresh animation of Ollie breaking out of his egg, only to flap his wings as my feed loads. Again, this is something wholly unnecessary, but it sure is delightful, and I love it.

John Gruber shared similar sentiments in his review from 2011 of Tweetbot 1.0:

I greatly appreciate the exquisite attention to detail that went into every single pixel of this app. E.g., they even created their own custom alert windows. Try entering the wrong password for a Twitter account and watch the animation. That's joyful UI design.

Key last sentence. The superfluousness is joyful — not only to users, but developers as well.